Ultrasound imaging has provided useful information about the interior characteristics of an object or subject under examination. An ultrasound scanner has included a transducer array with transducer elements that transmit ultrasonic waves into a field of view and that receive echoes reflected from structure in the field of view. The echoes are processed, generating an image of the structure and field of view. Transducer elements include a piezoelectric transducer (PZT) element, a ceramic element, and Capacitive Micro-machined Ultrasound Transducer (CMUT) element.
With a CMUT element, energy transduction in receive mode corresponds to a change in capacitance of a capacitive element therein. For transmit, a suitable applied bias voltage causes electrostatic forces that vibrate the capacitive element, which produces ultrasonic waves. For receive, echoes incidence on the capacitive element modulate the capacitance of the capacitive element, invoking generation of a corresponding voltage. For transmit and receive, a high bias voltage will result in a high sensitivity/conversion efficiency. FIGS. 1 and 2 illustrated two different prior art biasing approaches.
FIG. 1 depicts front end electronics 1021, . . . , 102N for a CMUT element 1041, . . . , 104N, where N is a positive integer. Each front end includes a transmit channel 1061, . . . , 106N and a receive channel 1081, . . . , 108N. The transmit channel 1061, . . . , 106N include an anti-pole diode 1101, . . . , 110N that passes a high voltage transmit pulse from transmit circuitry (TX) during transmit and acts as an open circuit during receive. Each front end further includes an amplifier 1121, . . . , 112N that amplifies and routes CMUT element signals to receive circuitry (RX) during receive. The amplifier 1121, . . . , 112N is referenced to ground 122 through resistive element 1241, . . . , 124N. The transmit signal can also be referenced to ground 122. The input signal is maintained below ±0.6V for the amplifier 1121, . . . , 112N to function properly.
A bias-network 1141, . . . , 114N includes a resistive element 1161, . . . , 116N and a CMUT element 1041, . . . , 104N, arranged as an RC network, or a low pass filter. The resistive element 1161, . . . 116N isolates the CMUT element 1041, . . . , 104N from a bias voltage 120 at the working frequency, while allowing the bias voltage 120 to charge the CMUT element 1041, . . . , 104N to the DC bias potential. A DC-blocking capacitor 1181, . . . , 118N is located between both the CMUT element 1041, . . . , 104N and the amplifier 1121, . . . , 112N and the anti-pole diode 1101, . . . , 110N. The bias voltage 120 is applied to each bias-network 1141, . . . , 114N, and each CMUT element 1041, . . . , 104N is electrically connected to a same electrical ground 122.
The sensitivity/conversion efficiency of the CMUT element 1041, . . . , 104N can be increased by increasing the bias voltage 120. However, generally, the resistive element 1161, . . . , 116N and the capacitive element 1181, . . . , 118N are electrically rated for the bias voltage 120. As such, in order to increase the sensitivity/conversion efficiency through increasing the bias voltage, the resistive element 1161, . . . , 116N and the capacitive element 1181, . . . , 118N would need to be higher rated components, which may add cost and physical space.
FIG. 2 is similar except the bias-network 1141, . . . , 114N is omitted, the bias voltage 120 is directly connected to the CMUT 1041, . . . , 104N, and the CMUT 1041, . . . , 104N is directly connected to the amplifier 1121, . . . , 112N. Unfortunately, a fault in a single CMUT element 1041, . . . , 104N will apply the full bias voltage 120 to the amplifier 1121, . . . , 112N (an electrical short), which can damage circuitry of the scanner, or fault the bias voltage 120 (an open circuit), which may fault the entire transducer. This configuration may also require a very low noise bias voltage, as there is no filtering performance of a bias resistive element like the resistive element 1161, . . . , 116N.